The present invention relates to a printer of a type in which a hammer bank having a plurality of spring steel print hammers, each of which is provided with a print pin at one end thereof, is reciprocated along a printing line while the hammers are selectively activated to effect printing. Each of the print hammers is rigidly supported at one end. In its non-printing state, the other end, that is, the end bearing the print pin, is held in a retracted position by the magnetic force of a permanent magnet. To release the print hammer to cause it to be propelled toward the recording medium under its own spring force to thus print a dot, a release coil is activated which produces a magnetic field which opposes that of the permanent magnet. An example of such a printer system is described in U.S. Pat. No. 3,941,051 to Barrus et al. and illustrated in FIG. 1.
As shown in FIG. 1, a print hammer unit of the prior art dot matrix printer includes a leaf spring 5 to the front surface of which are fixed a print pin 6 and a plunger 7 made of a magnetic material. Together, the leaf spring 5, pin 6 and plunger 7 constitute a print hammer. The print hammer unit further includes a permanent magnet 1, a front yoke 2, a rear yoke 3 and an electromagnetic coil 4. The electromagnetic coil 4 is wound around an upper leg portion of the rear yoke 3, that is, a pole portion 30. The lower end of the leaf spring 5 is fixed to a spacer 8 provided at the front of the permanent magnet 1 by means of a screw 9 which passes through the front yoke 2. Reference numeral 10 denotes a screw which fixes the leaf spring 5 to the spacer 8. A slot is formed in the upper end of the front yoke 2 through which the print pin 6 and the plunger 7 pass.
When the coil 4 is not energized, the upper end of the print hammer, including the print pin 6, is retracted against the pole portion 30 due to the magnetic force of the permanent magnet 1. In this position, the leaf spring 5 is elastically deformed. When the electromagnetic coil 4 is energized, the magnetic force generated by the permanent magnet is opposed and effectively cancelled, thereby releasing the leaf spring 5 and permitting it to be propelled by its own spring force toward the printing position and hence causing the print pin 6 to strike against a platen (not shown) through a printing paper and an ink ribbon (not shown). A dot is printed on the printing paper in this manner.
In the complete hammer bank (printer head), the permanent magnet 1, the yokes 2 and 3, etc. extend along substantially the whole printing line. The hammer bank is reciprocated along the printing line and the printing paper is advanced in a stepped manner, one step per vertical column of dots. As the hammer bank is reciprocated, the various electromagnetic coils are energized in such a manner as to cause the printing of dots in position corresponding to desired patterns to be printed, particularly, alphanumeric patterns. The hammer bank is reciprocated back and forth through a total distance corresponding, for instance, to six columns of dots (one character space).
A problem exists in the dot printer thus constructed. Specifically, in the case where more than a certain number of electromagnetic coils 4 are energized at one time, a delay in the release of some of the print hammer is encountered, thereby lowering the quality of printed characters. Such a problem is disclosed in U.S. Pat. No. 4,280,404 to Barrus et al.
Specifically, in the case where both print hammers adjacent a given hammer are simultaneously released and then the given print hammer is released, the release of the given print hammer will be delayed, again lowering the quality of the printed characters. This problem can be solved by increasing the time period for which the electromagnetic coils are energized. However, another problem is then encountered in that control circuitry becomes complicated and the heat generated by the coils becomes excessive.
It is believed that the above-described phenomenon is attributed to the fact that a portion of the magnetic flux which flows in the pole portions 30 in two previously activated hammer units on either side of a given hammer unit strays into the pole portion 30 of the given hammer unit, causing a momentary increase in the magnetic attractive force, and hence delaying the cancellation of the permanent-magnet-generated magnetic field by the energization of the electromagnetic coil 4 associated with the given hammer. It is possible to prevent this delay by extending the period of energization of the associated electromagnetic coil 4, that is, energizing it starting at an earlier time. However, the same problems mentioned above related to the complexity of the control circuitry needed to implement the technique and the generation of heat occur.